1. Field of the Invention
The present invention relates to a method for securing a memory area comprising data blocks, and an electronic circuit comprising groups of memory cells each receiving a data block, and means for executing a command for writing a data block in a group of memory cells.
The present invention relates to the securization of a binary counter implemented on an integrated circuit, such as integrated circuits for odometers of motor vehicles.
The present invention also relates to the securization of integrated circuits used to manage transaction data, such as integrated circuits for smart cards of electronic cash card, telephone card, transportation pass, etc.
2. Description of the Related Art
Transaction or counting data generally have a tangible monetary value, whether the data is transaction data enabling a service to be accessed or counting data of an odometer enabling the value of a vehicle to be established. As a result, the integrated circuits comprising such data are often the target of fraudsters who try to falsify or refresh these data.
The memories used to store this type of data are non-volatile memories generally of the electrically programmable and erasable type. Thus, a fraudster who wishes to refresh the value of a transaction unit counter or of a binary counter will generally try to erase memory cells that are in the programmed state so that all or part of the units of the counter are reset to 0. The erasing technique most frequently used is the collective erasing of the memory cells, by exposing the memory to a beam of particles, generally a UV beam (beam of ultra-violet light). The exposure to the UV light results in electric charges being extracted from the memory cells and put into a blank electrical state corresponding generally, when read, to the erased state.
A solution to counter this type of fraud, described by EP 1 006 532, is to provide a control memory cell that is put into an initial, erased or programmed state, and is then read with two read voltages so as to detect whether the memory cell is in the initial state or in an intermediate state corresponding to the UV state. This solution has the disadvantage of needing two read voltages, one being a “standard” read voltage and the other one allowing whether or not the memory cell is in the UV state to be detected.
Moreover, this solution does not counter certain types of fraud such as the corruption of the read process, which enables protection mechanisms intervening at the time of reading data to be inhibited, or the corruption of the writing process, enabling counterfeit data to be written in a memory.